Sulfur dioxide is a common contaminant of industrial plant vent gas streams and flue gases. It is also found in the vent gases from many ore smelting processes. The venting of gas streams containing sulfur dioxide to the atmosphere constitutes an undesirable pollution of the environment.
Various methods have been developed to reduce the emission of sulfur dioxide to the atmosphere and to recover the sulfur values thereof as elemental sulfur or other marketable sulfur products. One general method by which sulfur dioxide may be removed from a vent gas stream is by absorbing the sulfur dioxide in an absorption solution prior to venting the gas stream to the atosphere. Subsequently, the sulfur dioxide-loaded absorption solution may be regenerated to liberate a concentrated sulfur dioxide gas stream which is suitable for further treatment by known methods for the recovery of the sulfur values thereof as elemental sulfur, sulfuric acid as other marketable sulfur product. One such known method comprises the concentration of a dilute sulfur dioxide gas stream in an appropriate absorption-regeneration process to provide a concentrated sulfur dioxide gas stream suitable as feed to a Claus process for conversion to elemental sulfur.
In most instances industrial plant vent gases which contain sulfur dioxide also contain other acid gas components, especially carbon dioxide, from which it is desirable to separate sulfur dioxide prior to any further treatment process for the recovery of sulfur values. Carbon dioxide is a major diluent of flue and smelter off gases. Claus plant tail gases generally contain considerable quantities of carbon dioxide as well as sulfur dioxide. To be practical for removing sulfur dioxide and concentrating it for further treatment, a sulfur dioxide absorption process should be capable of preferentially absorbing sulfur dioxide in the presence of carbon dioxide.
Several absorption-regeneration processes for concentration of sulfur dioxide are known which employ various sulfur dioxide absorption solvents, such as solutions of sodium carbonate, sodium hydroxide, ammonium hydroxide, aqueous ammonia, disodium hydrogen phosphate, alkanolamines, trialkanolamines, and others. U.S. Pat. No. 3,904,735 discloses that aqueous trialkanolamines or tetrahydroxyalkyl alkylene diamines are selective sulfur dioxide absorption solvents. U.S. Pat. No. 3,767,777 discloses that a solvent comprising a water-soluble trialkyl phosphate containing at least about 5% water preferentially absorbs sulfur dioxide from a gas stream which also contains significant quantities of carbon dioxide.
Each of the above-described sulfur dioxide absorption solvents suffer from certain disadvantages which adversely effect their use in a commercial process. One disadvantage is that nonregenerable heat stable salts, particularly sulfate salts, accumulate in such solvents over prolonged periods of operation. To preserve the sulfur dioxide absorption capacity of such solvents, the accumulated heat stable salts, particularly the sulfates, must be purged from the system. A second disadvantage which pertains to the use of water-soluble trialkyl phosphates as a preferential sulfur dioxide absorption solution is their tendency to degrade during operation. Trialkyl phosphate (TAP) solvents which contain at least about 5% water have been found to degrade into other compounds which are not capable of sulfur dioxide absorption. For this reason, trialkyl phosphate solvents have not been widely employed in any commercial application for the removal of sulfur dioxide from industrial plant vent gas streams.